In the electrode catalyst of conventional polymer electrolyte fuel cells (PEFC), the quantity of platinum to be used is about 1 g Pt/kW for fuel cell vehicles and 5 to 8 g Pt/kW for stationary use. This implies that a cost of hundreds of thousands of yen/vehicle is necessary for a platinum metal of the electrode catalyst of FCV in consideration of the present commercial price of platinum. The present target concerning the quantity of platinum to be used in FCV is to reduce the quantity of platinum to about 1/10 that used at present, that is, about 0.1 g/kW for vehicles and about 1 g/kW for stationary use by the time FCVs initially spread which is expected in 2015 to 2020. This needs a further improvement in mass activity (A/g), that is, the amount of current which can be extracted from platinum per unit mass.
Further, when a carbon-supported catalyst is used as the catalyst used in FCV, the carbon support of the carbon-supported catalyst is severely corroded at the start or stop of operations, which promotes the deterioration of the catalyst layer and further the deterioration of the membrane electrode assembly. For this, it is required to improve the catalyst layer to thereby largely improve the durability, and particularly, the durability of the cycle durability.
There is a method for forming the catalyst layer by the sputtering method or vapor deposition method. Such an electrode ensures higher durability as compared with an electrode using a carbon-supported catalyst. Specifically, a platinum catalyst material is deposited on a whisker substrate by sputtering. According to this method, high durability is obtained and also, the amount of platinum to be used can be limited. However, a platinum catalyst block having a size of tens of nanometers is formed in this method, and therefore, this method is inadequate in utilization efficiency of a catalyst material.
Further, there is a method in which a catalyst material mixed with a pore-forming material is used to form a catalyst layer, and then, the pore-forming material is melted to remove it, thereby forming pores in the catalyst layer. Specifically, a mixed layer and a pore-forming material layer constituted of a catalyst material and a pore-forming material are formed by sputtering and then, the pore-forming material in the mixed layer and pore-forming material layer are melted to remove them, thereby forming a laminate structure including a catalyst coagulation layer and a void layer such that high catalyst utilization efficiency is attained. However, this method is unsatisfactory in view of durability or the like.